The Quatermass Xperiment (The Creeping Unknown in the United States) is a 1955 British science-fiction/horror film.

The plot of the film involves the return to Earth of astronaut Victor Carroon, who has become the first man in space in a rocket ship of Quatermass's design. However, it becomes very clear that something infected Carroon during the flight, and he rapidly begins mutating into an alien organism which, if it spores, will destroy humanity. Quatermass and his associates have just a few hours to track the creature down and prevent an apocalypse.